USB DMA异常现象，请教

star316

USB 传输完成后， 在 write_callback 中调用
usb_buffer_free(urb->dev,  urb->transfer_buffer_length, urb->transfer_buffer,
  urb->transfer_dma);  有时侯会出现WARNING ， 只是间歇出现的， 请问是什么问题？

WARNING: at /vobs/oms_moto_linux/linux-2.6.29/arch/arm/mm/dma-mapping.c:368 dma_free_coherent+0x34/0x304()

(dump_stack+0x0/0x14) from [<c0070158>] (warn_slowpath+0x70/0x8c)
(warn_slowpath+0x0/0x8c) from [<c004329c>] (dma_free_coherent+0x34/0x304)
(dma_free_coherent+0x0/0x304) from [<c026d85c>] (hcd_buffer_free+0x80/0x8
（hcd_buffer_free+0x0/0x8 from [<c02614a4>] (usb_buffer_free+0x2c/0x30)
(usb_buffer_free+0x0/0x30) from [<c0280f74>] (ipc_write_callback+0xbc/0xf0)